On Jun. 11, 2009 the World Health Organization raised the global pandemic alert level to phase 6, the pandemic phase, in response to the emergence and global spread of a novel influenza A (H1N1) virus henceforth referred to as 2009 A/H1N1 virus containing a unique combination of genes of swine origin (Garten et al., “Antigenic and Genetic Characteristics of Swine-Origin 2009 A(H1N1) Influenza Viruses Circulating in Humans” Science, May 22, 2009, Science Express Online; incorporated herein by reference). The emergence of this new strain can be traced back to March-April 2009 when there were reports of increased numbers of patients with influenza-like illness and associated hospitalizations and deaths in several areas of Mexico. In the period Mar.-Jun. 19, 2009, there have been over 44,000 laboratory-confirmed human cases of influenza 2009 A/H1N1 infections reported in 85 countries on 6 continents.
Given the rapid outbreak of this virus, there have been uncertainties associated with its virulence, transmissibility and its origins. Recent studies using epidemiological models to interpret data on outbreaks of the 2009 H1N1 virus have indicated that its transmissibility is on the lower end of what was estimated for the 1918 H1N1 pandemic. In most of the cases of human infection with the 2009 H1N1 viruses, the symptoms have been relatively mild, however, there have been over 150 deaths reported. Additionally, a substantial portion (˜40%) of infected individuals experience gastrointestinal distress and vomiting, a percentage that is higher than what is typically observed for seasonal flu.